Organometal halide perovskite has emerged as a promising material for solar cells and optoelectronics. Although the long diffusion length of photogenerated carriers is believed to be a critical factor responsible for the material's high efficiency in solar cells, a direct study of carrier transport over long distances in organometal halide perovskites is still lacking. We fabricated highly oriented crystalline CH3NH3PbI3 (MAPbI3) thin-film lateral transport devices with long channel length (∼120 μm). By performing spatially scanned photocurrent imaging measurements with local illumination, we directly show that the perovskite films prepared here have very long transport lengths for photogenerated carriers, with a minority carrier (electron) diffusion length on the order of 10 μm. Our approach of applying scanning photocurrent microscopy to organometal halide perovskites may be further used to elucidate the carrier transport processes and the vastly different carrier diffusion lengths (∼100 nm to 100 μm) in different types of organometal halide perovskites.